Abstract
AbstractThe use of stable isotope analysis to resolve ecosystem respiration into its plant and soil components rests on how well the end‐member isotope signatures (δ13C) are characterised. In general, it is assumed that end‐member values are constant over time. However, there are necessarily diurnal and other transient variations in end‐members with environmental conditions. We analyse diurnal and seasonal patterns of ecosystem respiration and its δ13C in a C4 grass growing in a C3 soil using fixed and diurnally varying plant and soil δ13C end‐members. We measure the end‐members independently, and we assess the effects of expected variation in values. We show that variation in end‐members within realistic ranges, particularly diurnal changes in the plant end‐member, can cause partitioning errors of 40% during periods of high plant growth. The effect depends on how close the end‐member is to the measured net respiration δ13C, that is, the proportion of the respiration due to that end‐member. We show light‐driven variation in plant end‐members can cause substantial distortion of partitioned soil organic matter (SOM) flux patterns on a diurnal scale and cause underestimation of daily to annual SOM turnover of approximately 25%. We conclude that, while it is not practicable to independently measure the full temporal variation in end‐member values over a growing season, this error may be adjusted for by using a diurnally varying δ13Cplant.Highlights End‐member δ13C values used to partition ecosystem respiration vary diurnally and seasonally Patterns of ecosystem respiration and its δ13C in a C4 grass growing in a C3 soil were analysed. Ignoring temporal changes in end‐member δ13C values can cause large errors in partitioning Long‐term data sets with sufficient temporal resolution can be used to correct for this
Highlights
The effects of plant roots and rhizodeposition on soil organic matter (SOM) turnover remain among the most poorly understood aspects of the terrestrial carbon (C) cycle (Hartmann et al, 2020)
Patterns of ecosystem respiration and its δ13C in a C4 grass growing in a C3 soil were analysed
In principle, partitioning errors on diurnal and other short timescales can affect both the inferred short-term variation in SOM fluxes and longer-term seasonal and annual dynamics and resulting conclusions about plant and soil processes. We explore this with a data set of diurnal and seasonal patterns of C fluxes and δ13C in a C4 plant–C3 soil field system
Summary
The effects of plant roots and rhizodeposition on soil organic matter (SOM) turnover remain among the most poorly understood aspects of the terrestrial carbon (C) cycle (Hartmann et al, 2020). Larger differences can be created, either by labelling plants with CO2 enriched or depleted in 13C, or, more practicably under field conditions, by exploiting the large δ13C difference between C3 and C4 photosynthetic pathways, which is typically 10‰–20‰ (Balesdent et al, 1987; Farquhar et al, 1989). This may be done by growing a C4 plant in a soil that has previously only hosted C3 plants, or vice-versa (Rochette & Flanagan, 1997; Wang et al, 2016; Xiao et al, 2015), or with natural δ13C gradients across transects of C3–C4 vegetation (Millard et al, 2008). The δ13C of SOM respiration may differ from that of the bulk SOM and may vary
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